Extensive characterization of genetic alterations in a series of human colorectal cancer cell lines

Biosensor-Based Micro-Affinity Purification for the Proteomic Analysis of Protein Complexes

A biosensor-based micro-affinity purification method to recover protein binding partners and their complexes for down stream proteomics analysis has been developed using the BIAcore 3000 fitted with a prototype Surface Prep Unit (SPU). The recombinant GST-intracellular domain of E-cadherin or the recombinant GST-beta-catenin binding domain of Adenomatous Polyposis Coli (APC) were immobilized onto the SPU and used to affinity purify binding partners from chromatographically enriched SW480 colon cancer cell lysates. A GST- immobilized surface was used as a control. Samples recovered from the SPU were subjected to SDS-PAGE with sensitive Coomassie staining followed by automated in-gel digestion and LC-MS/MS. The results obtained using the SPU were compared with similar experiments performed using Sepharose beads.

Deregulated GSK3beta activity in colorectal cancer: its association with tumor cell survival and proliferation

Glycogen synthase kinase 3beta (GSK3beta) reportedly has opposing roles, repressing Wnt/beta-catenin signaling on the one hand but maintaining cell survival and proliferation through the NF-kappaB pathway on the other. The present investigation was undertaken to clarify the roles of GSK3beta in human cancer. In colon cancer cell lines and colorectal cancer patients, levels of GSK3beta expression and amounts of its active form were higher in tumor cells than in their normal counterparts; these findings were independent of nuclear accumulation of beta-catenin oncoprotein in the tumor cells. Inhibition of GSK3beta activity by phosphorylation was defective in colorectal cancers but preserved in non-neoplastic cells and tissues. Strikingly, inhibition of GSK3beta activity by chemical inhibitors and its expression by RNA interference targeting GSK3beta induced apoptosis and attenuated proliferation of colon cancer cells in vitro. Our findings demonstrate an unrecognized role of GSK3beta in tumor cell survival and proliferation other than its predicted role as a tumor suppressor, and warrant proposing this kinase as a potential therapeutic target in colorectal cancer.

Differential nonsense mediated decay of mutated mRNAs in mismatch repair deficient colorectal cancers

The nonsense-mediated decay (NMD) system normally targets mRNAs with premature termination codons (PTCs) for rapid degradation. We investigated for a putative role of NMD in cancers with microsatellite instability (MSI-H cancers), because numerous mutant mRNAs containing PTC are generated in these tumors as a consequence of their mismatch repair deficiency. Using a quantitative RT-PCR approach in a large series of colorectal cancer cell lines, we demonstrate a significantly increased rate of degradation of mutant mRNAs containing a PTC compared with wild-type. A specific siRNA strategy was used to inhibit RENT-1 and/or RENT-2 activity, two major genes in the NMD system. This allowed us to show that increased degradation of PTC-containing mRNAs in MSI-H tumors was partly dependent upon NMD activity. The efficiency of NMD for the degradation of mutant mRNAs from target genes was highly variable in these cancers. NMD degraded some of them (TGFssRII, MSH3, GRK4), although allowing the persistent expression of others (BAX, TCF-4). This is of particular interest within the context of a proposed conservation of biological activity for the corresponding mutated proteins. We thus propose that NMD might play an important role in the selection of target gene mutations with a functional role in MSI-H carcinogenesis.

Genetic determinants of methotrexate responsiveness and resistance in colon cancer cells

Alternative genetic pathways characterized by specific genetic profiles and exhibiting distinctive biological and clinical features have been proposed in colorectal carcinogenesis. Methotrexate (MTX) is a potent inhibitor of the dihydrofolate reductase (DHFR) enzyme, which is essential for DNA synthesis and cell growth. We have evaluated the association between different genetic features and the capacity to develop MTX resistance in colon cancer cell lines representative of alternative genetic pathways. Three aneuploid cell lines (HT-29, SW480, and SK-CO-1) showed pre-existing amplifications, but only one (HT-29) developed MTX resistance, showing amplification of the DHFR gene at 5q12-14 (>20-fold amplification and presence of extrachromosomal double minutes). Failure to develop resistance was attributed to the absence of two complete chromosomes 5 in SW480 and SK-CO-1 cells. Four near-diploid cell lines (LoVo, HCT116, DLD-1 and KM12C) and two aneuploid KM12C-derived metastases (KM12SM and KM12L4A) developed MTX resistance but none exhibited DHFR amplification. All resistant cells without DHFR gene amplification showed microsatellite instability. We conclude that chemoresistance capacity and the mechanism of chemoresistance are related with the genetic pathway and the karyotypic features of colon cancer cells.

Mononucleotide repeats BAT-26 and BAT-25 accurately detect MSI-H tumors and predict tumor content: implications for population screening

Tumors with a defective DNA mismatch repair system (MSI-H tumors) have distinct molecular and clinicopathologic profiles compared with mismatch repair-proficient tumors and are associated with a relatively favorable prognosis. There is evidence to suggest that colorectal cancer patients with MSI-H tumors respond differently to adjuvant chemotherapy. Determination of MSI status also has clinical application for assisting in the diagnosis of suspected hereditary nonpolyposis colorectal cancer cases. For these reasons, it is becoming increasingly apparent that testing for MSI should be conducted routinely in human cancer types that frequently present with such a phenotype. BAT-26 and BAT-25 are mononucleotide repeats that are widely used to establish the MSI status of human tumors. We show here that their allelic size profiles provide an estimate of the percentage of contaminating normal cells in MSI-H tumors. These markers are sensitive enough to detect instability when the tumor cell content of a sample is as low as 5-10%. MSI-H tumors contain mutations in coding repeats within genes known to be targets for instability. In cases with low tumor cell content, no mutations in any of 9 coding repeats were detected. However, when these samples were enriched for tumor cells, mutations were detected in the same target genes. Thus, BAT-26 and BAT-25 markers accurately identify MSI-H tumors without prior need for enrichment for tumor cells and indicate which samples require further purification before screening for mutations in target genes for instability. Our results have implications for large-scale screening of cancer patients to determine MSI-H status and prognosis.

Genome signatures of colon carcinoma cell lines

In cancer biology, cell lines are often used instead of primary tumors because of their widespread availability and close reflection of the in vivo state. Cancer is a genetic disease, commonly caused by small- and large-scale DNA rearrangements. Therefore, it is essential to know the genomic profiles of tumor cell lines to enable their correct and efficient use as experimental tools. Here, we present a comprehensive study of the genomic profiles of 20 colon cancer cell lines combining conventional karyotyping (G-banding), comparative genomic hybridization (CGH), and multicolor fluorescence in situ hybridization (M-FISH). Major differences between the microsatellite instability (MSI) and chromosome instability (CIN) cell lines are shown; the CIN cell lines exhibited complex karyotypes involving many chromosomes (mean: 8.5 copy number changes), whereas the MSI cell lines showed considerably fewer aberrations (mean: 2.6). The 3 techniques complement each other to provide a detailed picture of the numerical and structural chromosomal changes that characterize cancer cells. Therefore, 7 of the cell lines (Colo320, EB, Fri, IS2, IS3, SW480, and V9P) are here completely karyotyped for the first time and, among these, 5 have not previously been cytogenetically described. By hierarchical cluster analysis, we show that the cell lines are representative models for primary carcinomas at the genome level. We also present the genomic profiles of an experimental model for tumor progression, including 3 cell lines (IS1, IS2, and IS3) established from a primary carcinoma, its corresponding liver- and peritoneal metastasis from the same patient. To address the question of clonality, we compared the genome of 3 common cell lines grown in 2 laboratories. Finally, we compared all our results with previously published CGH data and karyotypes of colorectal cell lines. In conclusion, the large variation in genetic complexity of the cell lines highlights the importance of a comprehensive reference of genomic profiles for investigators engaged in functional studies using these research tools.

Comparative genomic hybridization using oligonucleotide microarrays and total genomic DNA

Array-based comparative genomic hybridization (CGH) measures copy-number variations at multiple loci simultaneously, providing an important tool for studying cancer and developmental disorders and for developing diagnostic and therapeutic targets. Arrays for CGH based on PCR products representing assemblies of BAC or cDNA clones typically require maintenance, propagation, replication, and verification of large clone sets. Furthermore, it is difficult to control the specificity of the hybridization to the complex sequences that are present in each feature of such arrays. To develop a more robust and flexible platform, we created probe-design methods and assay protocols that make oligonucleotide microarrays synthesized in situ by inkjet technology compatible with array-based comparative genomic hybridization applications employing samples of total genomic DNA. Hybridization of a series of cell lines with variable numbers of X chromosomes to arrays designed for CGH measurements gave median ratios for X-chromosome probes within 6% of the theoretical values (0.5 for XY/XX, 1.0 for XX/XX, 1.4 for XXX/XX, 2.1 for XXXX/XX, and 2.6 for XXXXX/XX). Furthermore, these arrays detected and mapped regions of single-copy losses, homozygous deletions, and amplicons of various sizes in different model systems, including diploid cells with a chromosomal breakpoint that has been mapped and sequenced to a precise nucleotide and tumor cell lines with highly variable regions of gains and losses. Our results demonstrate that oligonucleotide arrays designed for CGH provide a robust and precise platform for detecting chromosomal alterations throughout a genome with high sensitivity even when using full-complexity genomic samples.

Epigenetic inactivation of RUNX3 in microsatellite unstable sporadic colon cancers

Runt domain transcription factors are important targets of TGF-beta superfamily proteins and play a crucial role in mammalian development. Three mammalian runt-related genes, RUNX1, RUNX2 and RUNX3, have been described. RUNX3 has been shown to be a putative tumor suppressor gene localized to chromosome 1p36, a region showing frequent loss of heterozygosity events in colon, gastric, breast and ovarian cancers. Because of the important role of TGF-beta signaling in the human colon, we hypothesized that RUNX3 may serve as a key tumor suppressor in human colon cancers and colon cancer-derived cell lines. We examined RUNX3 expression and the frequency of RUNX3 promoter hypermethylation in 17 colon cancer cell lines and 91 sporadic colorectal cancers. Semiquantitative analysis of RUNX3 transcripts was performed by RT-PCR and de novo methylation of the RUNX3 promoter was studied by a methylation-specific PCR (MSP) assay. Nineteen of 91 informative tumors (21%) and 11 of 17 (65%) colon cancer cell lines exhibited hypermethylation of the RUNX3 promoter. Interestingly, RUNX3 promoter hypermethylation was more common in tumors exhibiting high frequency of microsatellite instability (MSI-H) (33% of MSI-H vs. 12% of MSI-L/MSS tumors; p = 0.012). Hypermethylation of the RUNX3 promoter correlated with loss of mRNA transcripts in all cell lines. RUNX3 promoter methylation was reversed and its expression restored in SW48 and HCT15 colon cancer cells after treatment with the demethylating agent 5-aza-2'-deoxycytidine, indicating that loss of expression is caused by epigenetic inactivation in colon carcinogenesis. This is the first demonstration of frequent de novo hypermethylation of the RUNX3 promoter in sporadic colon cancers. The significant association of RUNX3 promoter hypermethylation with MSI-H colon cancers suggests that RUNX3 is a novel target of methylation, along with the hMLH1 gene, in the evolution of MSI-H colorectal cancers.

E-Cadherin (CDH1) and p53 rather than SMAD4 and Caspase-10 germline mutations contribute to genetic predisposition in Portuguese gastric cancer patients

Approximately 30% of all hereditary diffuse gastric cancer (HDGC) families carry CDH1 germline mutations. The other two thirds remain genetically unexplained and are probably caused by alterations in other genes. Using polymerase chain reaction (PCR)/single-strand conformation polymorphism (SSCP)/sequencing, we screened 32 Portuguese families with a history of gastric cancer and 23 patients with early onset gastric cancer for CDH1 germline mutations. In probands negative for CDH1 mutations, we screened genes involved in hereditary cancer syndromes in which gastric cancer may be one of the component tumours, namely p53 (Li-Fraumeni Syndrome) and hMLH1 and hMSH2 (HNPCC). We also screened in these patients for mutations in Caspase-10, a gene inactivated in sporadic gastric cancer, and SMAD4, a gene whose inactivation in mice is associated with signet-ring cell carcinoma of the stomach. One of the families fulfilling the HDGC criteria harboured a CDH1 germline mutation, and one of the families with incomplete criteria harboured a p53 germline mutation. No mutations were identified in hMLH1 and hMSH2, and only sequence variants were found in SMAD4 and Caspase-10. The present work reports for the first time CDH1 germline mutations in Portuguese gastric cancer families, and highlights the need for p53 mutation screening in families lacking CDH1 germline mutations, in a country with one of the highest incidences of gastric cancer in the world. No evidence was found for a role of germline mutations in SMAD4 and Caspase-10 in families lacking CDH1 mutations.

A CpG island hypermethylation profile of primary colorectal carcinomas and colon cancer cell lines

Background

Tumor cell lines are commonly used as experimental tools in cancer research, but their relevance for the in vivo situation is debated. In a series of 11 microsatellite stable (MSS) and 9 microsatellite unstable (MSI) colon cancer cell lines and primary colon carcinomas (25 MSS and 28 MSI) with known ploidy stem line and APC, KRAS, and TP53 mutation status, we analyzed the promoter methylation of the following genes: hMLH1, MGMT, p16^INK4a (CDKN2A α-transcript), p14^ARF (CDKN2A β-transcript), APC, and E-cadherin (CDH1). We compared the DNA methylation profiles of the cell lines with those of the primary tumors. Finally, we examined if the epigenetic changes were associated with known genetic markers and/or clinicopathological variables.

Results

The cell lines and primary tumors generally showed similar overall distribution and frequencies of gene methylation. Among the cell lines, 15%, 50%, 75%, 65%, 20% and 15% showed promoter methylation for hMLH1, MGMT, p16^INK4a, p14^ARF, APC, and E-cadherin, respectively, whereas 21%, 40%, 32%, 38%, 32%, and 40% of the primary tumors were methylated for the same genes. hMLH1 and p14^ARF were significantly more often methylated in MSI than in MSS primary tumors, whereas the remaining four genes showed similar methylation frequencies in the two groups. Methylation of p14^ARF, which indirectly inactivates TP53, was seen more frequently in tumors with normal TP53 than in mutated samples, but the difference was not statistically significant. Methylation of p14^ARF and p16^INK4a was often present in the same primary tumors, but association to diploidy, MSI, right-sided location and female gender was only significant for p14^ARF. E-cadherin was methylated in 14/34 tumors with altered APC further stimulating WNT signaling.

Conclusions

The present study shows that colon cancer cell lines are in general relevant in vitro models, comparable with the in vivo situation, as the cell lines display many of the same molecular alterations as do the primary carcinomas. The combined pattern of epigenetic and genetic aberrations in the primary carcinomas reveals associations between them as well as to clinicopathological variables, and may aid in the future molecular assisted classification of clinically distinct stages.

Serum response factor is alternatively spliced in human colon cancer

BACKGROUND

Serum response factor (SRF) is a transcription factor that plays an important role in cellular differentiation and cell cycle regulation. SRF function is regulated in part by alternative splicing. Little is known about the expression or role of these alternatively spliced isoforms during tumorigenesis. We hypothesized that there is a change in the splice variants during intestinal tumorigenesis and that this change promotes the tumor phenotype.

MATERIALS AND METHODS

SRF expression was determined by Western blotting of benign intestinal cells and human colon cancer cell lines. To determine the effect of alternative splicing of SRF on intestinal growth and proliferation, the predominant alternatively spliced isoform of SRF that we identified in colon cancer cells, SRFDelta5, was transfected into IEC-6 cells. IEC-6 and IEC-6SRFDelta5 cells were plated and cell numbers were determined at four time points.

RESULTS

Western blotting demonstrates that full-length SRF is the predominant form of SRF in rat IEC-6 cells, normal human colonic mucosa, and HT-29 cells, derived from a well-differentiated human colonic adenocarcinoma. In the colon cancer cell lines derived from poorly differentiated tumors (WiDr, HCT 116, LoVo, and SW480), SRFDelta5 is the predominant isoform expressed. There was a significant increase in cell survival in IEC-6 cells transfected with SRFDelta5 compared to parental cells.

CONCLUSION

Our data demonstrate that an alternatively spliced isoform of SRF, SRFDelta5, is expressed in human colon cancer cell lines. Additionally, these data demonstrate that expression of SRFDelta5 may contribute to the tumor phenotype by affecting cell survival. This is the first study to document a change in expression of the alternatively spliced isoform of SRF in human malignancy.

Molecular analysis of APC promoter methylation and protein expression in colorectal cancer metastasis

APC (adenomatous polyposis coli) promoter methylation has been linked to the early development of colorectal cancers. However, the role of APC methylation and its effect on protein expression in colon cancer metastasis is largely unknown. In this study, we investigated APC promoter methylation by Methylight analysis and analysed the APC protein levels by immunohistochemistry and western blot analysis in 24 liver metastasis and 39 primary colorectal cancers. Promoter methylation of the APC gene was found to be a frequent event in liver metastasis (10/24) and significantly more frequent compared with primary colorectal cancer (7/39, P = 0.047). APC methylation was not found in 14 matched normal colon tissues. APC protein was detected in the cytoplasm of primary and metastatic cancer cells and non-tumorous colon epithelium. By western blot analysis, APC protein levels were found to be decreased in primary tumour tissues compared with the normal colon mucosa. In contrast, APC protein levels were not decreased in the cancer cells that had metastasized to the liver. APC protein levels were independent of the presence of APC promoter methylation or gene mutations. In summary, APC promoter methylation is a frequent epigenetic alteration in colorectal cancer metastasis. However, we observed no significant association between APC promoter methylation or gene mutation and APC protein expression in colorectal metastasis. Therefore, metastatic cancer cells seem to harbour a heterogenous genetic and epigenetic background, in which cancer cells may exhibit APC promoter methylation that is independent of APC expression.

Associations Among -TrCP, an E3 Ubiquitin Ligase Receptor, -Catenin, and NF- B in Colorectal Cancer

BACKGROUND

The ubiquitin-proteasome pathway is important in regulating protein signaling pathways that are involved in tumorigenesis. beta-transducin repeat-containing proteins (beta-TrCP) are components of the ubiquitin ligase complex targeting beta-catenin and IkappaBalpha for proteasomal degradation and are thus a negative regulator of Wnt/beta-catenin signaling and a positive regulator of NF-kappaB signaling. We analyzed expression of beta-TrCP in colorectal cancers and its association with types of beta-catenin subcellular localization, an indirect measure of activation.

METHODS

Levels of beta-TrCP1 mRNA and protein were measured by quantitative reverse transcription-polymerase chain reaction and immunoblotting, respectively, in samples of tumor and normal tissues from 45 patients with colorectal cancer. Types of beta-catenin activation (diffuse or invasion edge) and NF-kappaB activation were examined by immunohistochemistry. Apoptosis was determined by the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick-end labeling (TUNEL) assay. All statistical tests were two-sided.

RESULTS

Compared with the beta-TrCP1 levels in normal tissues, 25 (56%) of 45 tumors had increased beta-TrCP1 mRNA and protein levels. Of the 22 (49%) tumors with beta-catenin activation, 12 had the diffuse type (i.e., nuclear accumulation throughout the tumor) and 10 had the invasion edge type (i.e., nuclear accumulation predominantly in the tumor cells that formed the invasion edge). Increased beta-TrCP1 levels were statistically significantly associated with beta-catenin activation (P =.023) and decreased apoptosis (P =.035). beta-TrCP accumulated in the nuclei of tumor cells that contained increased levels of beta-TrCP1 mRNA and the active form of NF-kappaB. Higher levels of beta-TrCP1 mRNA were detected in primary tumors of patients who had metastases (0.960 arbitrary units, 95% confidence interval = 0.878 to 1.042) than in the tumors of patients who did not (0.722 arbitrary units, 95% confidence interval = 0.600 to 0.844; P =.016).

CONCLUSION

In colorectal cancer, increased expression of beta-TrCP1 is associated with activation of both beta-catenin and NF-kappaB, suggesting that the integration of these signaling pathways by increased beta-TrCP expression may contribute to an inhibition of apoptosis and tumor metastasis.

MEK1 and MEK2, different regulators of the G1/S transition

The ERK cascade is activated by hormones, cytokines, and growth factors that result in either proliferation or growth arrest depending on the duration and intensity of the ERK activation. Here we provide evidence that the MEK1/ERK module preferentially provides proliferative signals, whereas the MEK2/ERK module induces growth arrest at the G1/S boundary. Depletion of either MEK subtype by RNA interference generated a unique phenotype. The MEK1 knock down led to p21cip1 induction and to the appearance of cells with a senescence-like phenotype. Permanent ablation of MEK1 resulted in reduced colony formation potential, indicating the importance of MEK1 for long term proliferation and survival. MEK2 deficiency, in contrast, was accompanied by a massive induction of cyclin D expression and, thus, CDK4/6 activation followed by nucleophosmin hyperphosphorylation and centrosome over-amplification. Our results suggest that the two MEK subtypes have distinct ways to contribute to a regulated ERK activity and cell cycle progression.

Cdx1 inhibits human colon cancer cell proliferation by reducing beta-catenin/T-cell factor transcriptional activity

The cessation of proliferation and the induction of differentiation are highly coordinated processes that occur continuously in the intestinal crypts. The homeodomain transcription factors Cdx1 and Cdx2 regulate intestine-specific gene expression and enterocyte differentiation. Their roles in regulating proliferation are recognized but remain poorly understood. Previously, we demonstrated that Cdx1 expression diminished the proliferation of human colon cancer cells in part by reducing cyclin D1 gene expression. In order to elucidate further the molecular mechanisms underlying this phenomenon, we first hypothesized that Cdx1 or Cdx2 expression reduces colon cancer cell proliferation by inhibiting beta-catenin/T-cell factor (TCF) transcriptional activity. We report that Cdx1 or Cdx2 expression does inhibit beta-catenin/TCF transcriptional activity in colon cancer cells. This inhibitory effect is dose-dependent and is observed in different colon cancer cell lines, and the degree of inhibition correlates with the ability of Cdx1 to reduce cell proliferation. Cdx1 expression does not alter beta-catenin protein levels or intracellular distribution nor does it induce an inhibitory TCF isoform. We also find that Cdx1 expression is lost in Min mouse polyps with increased nuclear localization of beta-catenin, suggesting that Cdx1 does not support beta-catenin-mediated transformation. Finally, we show that colon cancer cells effectively reduce Cdx2-mediated inhibition of Wnt/beta-catenin/TCF transcriptional activity when compared with other model systems. This suggests that colon cancer and possibly crypt epithelial cells can modulate the effects of Cdx2 on beta-catenin signaling and proliferation. We conclude that Cdx1 and Cdx2 inhibit colon cancer cell proliferation by blocking beta-catenin/TCF transcriptional activity.

Reduction of menin expression enhances cell proliferation and is tumorigenic in intestinal epithelial cells

Menin, the product of the tumor suppressor gene MEN1, is widely expressed in mammalian endocrine and non-endocrine tissues, including intestine. Its known abundant expression in several types of cells with high proliferative capacity led us to investigate the physiological function of the protein menin in intestinal epithelium, one of the most rapidly growing epithelia. Here we showed that the Men1 gene is mainly expressed in the crypt compartment of the proximal small intestine and that its expression was increased during fasting in vivo, both suggesting a role of menin in the control of cell growth. Indeed, specific reduction of menin expression by transfected antisense cDNA in the rat duodenal crypt-like cell line, IEC-17, increased cell proliferation. The latter is correlated to a loss of cell-cycle arrest in G(1) phase by resting cells and an overexpression of cyclin D1 and cyclin-dependent kinase (Cdk)-4. Furthermore, these cells lost the inhibition of proliferation induced by transforming growth factor-beta1, associated with a decrease of transforming growth factor-beta type II receptor expression. As a result of deregulated proliferation, antisense menin transfected IEC-17 cells became tumorigenic as shown in vitro as well as in vivo in immunosuppressed animals. These results indicate that menin contributes to proliferation control in intestinal epithelial cells. The present study reveals an unknown physiological function for menin in intestine that may be important in the regulation of epithelial homeostasis.

Frequent loss of the AXIN1 locus but absence of AXIN1 gene mutations in adenocarcinomas of the gastro-oesophageal junction with nuclear beta-catenin expression

Up to 60% of gastro-oesophageal junction (GEJ) adenocarcinomas show nuclear β-catenin expression, pointing to activated T-cell factor (TCF)/β-catenin-driven gene transcription. We demonstrate in five human GEJ adenocarcinoma cell lines that nuclear β-catenin expression indeed correlates with enhanced TCF-mediated transcription of a reporter gene. In several tumour types, TCF/β-catenin activation is caused by mutations in either adenomatous polyposis coli (APC), β-catenin exon 3, AXIN1, AXIN2 or β-transducin repeat-containing protein (β-TrCP). In GEJ adenocarcinomas, very few APC and β-catenin mutations have been found. Therefore, the mechanism of Wnt pathway activation remains unclear. In the present study, we did not find AXIN1 gene mutations in 17 GEJ tumours with nuclear β-catenin expression (without β-catenin exon 3 mutations). Six intragenic single nucleotide polymorphisms (SNPs) were identified. One of these, the AXIN1 gene T1942C SNP, has a frequency of 21% but is only very recently described despite numerous AXIN1 gene mutational studies. We provide evidence why this SNP was missed in single strand conformation polymorphism analyses. The AXIN1 gene G2063A variation was previously described as a gene mutation but we demonstrate that this is a polymorphism. With these six SNPs loss of heterozygosity (LOH) was found in 11 of 15 (73%) informative tumours. To investigate a possible AXIN1 gene dosage effect in GEJ tumours expressing nuclear β-catenin, AXIN1 locus LOH was determined in 20 tumours expressing membranous and no nuclear β-catenin. LOH was found in 10 of 13 (77%) informative cases. AXIN1 protein immunohistochemistry revealed cytoplasmic expression in all tumours irrespective of the presence of AXIN1 locus LOH. These data indicate that nuclear β-catenin expression is indicative for activated Wnt signalling and that neither AXIN1 gene mutations nor AXIN1 locus LOH are involved in Wnt pathway activation in GEJ adenocarcinomas.

The Bmi-1 oncoprotein is overexpressed in human colorectal cancer and correlates with the reduced p16INK4a/p14ARF proteins

To clarify the roles of Bmi-1 in colorectal carcinoma, we examined the expression of Bmi-1 in 41 samples out of 46 colorectal carcinomas by reverse transcription-PCR, whereas all 46 were analyzed by immunostaining. In addition, we analyzed the expression patterns of Bmi-1 in association with p16INK4a and p14ARF (in mouse p19ARF) in a series of colorectal carcinomas. The level of Bmi-1 mRNA in the carcinoma tissues was significantly higher than those of the adjacent non-neoplastic colonic mucosal tissues. Immunohistochemistry for Bmi-1 showed moderate or strong expression levels in 65% (30/46) of colorectal carcinomas. Colorectal carcinomas with moderate or strong Bmi-1 expression were more likely to have low levels of the INK4 locus proteins (p16INK4a/p14ARF) (P<0.07). These results suggested that modulation of Bmi-1 protein might be involved in human colorectal carcinogenesis by repressing the INK4a/ARF proteins.

Activated BRAF targets proximal colon tumors with mismatch repair deficiency and MLH1 inactivation

BRAF, a serine/threonine kinase of the RAF family, is a downstream transducer of the RAS-regulated MAPK pathway and signals upstream of MEK1/2 kinases. Recently, activating mutations within BRAF have been reported in a high percentage of melanomas and colorectal carcinomas and shown to have oncogenic capabilities. Further, their association to mismatch-repair-deficient tumors has suggested the involvement of the RAS/RAF pathway in the tumorigenesis of microsatellite-unstable colon cancers, and that RAS and RAF mutations are alternative genetic events. We determined whether colorectal mismatch-repair-deficient tumors with BRAF mutations show a specific genotype when compared with tumors with wild-type BRAF, and whether they can be associated with a particular clinicopathological feature. Here, we report a striking association of BRAF, but not of APC, KRAS2, AXIN2, and TP53 mutations, with proximal mismatch-repair-deficient colon tumors and MLH1 hypermethylation. Our results support the hypothesis that proximal and distal colorectal tumors with mismatch repair deficiency harbor different genetic alterations, and we suggest that the involvement of the RAS/RAF pathway in colorectal tumorigenesis is differentially modulated according to tumor location and MLH1 inactivation.

Enhanced oxaliplatin-induced apoptosis following antisense Bcl-xl down-regulation is p53 and Bax dependent: Genetic evidence for specificity of the antisense effect

Introduction: Oxaliplatin, licensed for colorectal cancer chemotherapy, damages DNA by generating intrastrand and interstrand cross-links and can induce apoptosis via a Bax-dependent pathway. Bcl-xl, an antiapoptotic Bcl-2 family member, regulates apoptosis and chemoresistance in several cancer models. Bcl-xl expression correlates with invasiveness in primary colorectal cancer. Bcl-xl may therefore represent a therapeutic target in this disease. We used the mismatch repair-deficient HCT116 colorectal cancer cell line (wild-type HCT116) and p53 null, Bax null, or p21/WAF1 null derivatives to identify genetic determinants of the response to oxaliplatin and tested the hypothesis that antisense-mediated Bcl-xl down-regulation would enhance the apoptotic response in a p53- or Bax-dependent manner. Results: At clinically relevant concentrations, oxaliplatin induced p53 and p53-dependent Bax, Bcl-xl, and p21/WAF1 protein accumulation. A minor degree of apoptosis resulted via a p53- and Bax-dependent pathway. The major response was a transient mixed G1 and G2 growth arrest. The G1 arrest was p53 and p21/WAF1 dependent. A 2′-O-ribose methoxyethyl phosphorothioate antisense oligonucleotide reduced Bcl-xl protein expression by ∼90% in HCT116 (Bcl-xl knockdown). Missense controls were inactive. Prior Bcl-xl knockdown enhanced the apoptotic and the global cytotoxic effect of oxaliplatin. The extent of enhancement of apoptosis depended on the integrity of the p53- and Bax-mediated apoptotic pathway, providing genetic evidence that the desired proapoptotic antisense effect is due to specific down-regulation of the Bcl-xl target. Conclusion: The combination of oxaliplatin and Bcl-xl antisense merits testing in models of colorectal cancer in vivo.

Alterations of beta-catenin pathway in non-melanoma skin tumors: loss of alpha-ABC nuclear reactivity correlates with the presence of beta-catenin gene mutation

To determine the role of beta-catenin pathway in human skin carcinogenesis, 135 non-melanoma skin tumors were analyzed for beta-catenin expression and gene mutations. Intense nucleo-cytoplasmic immunoreactivity for C terminus beta-catenin antibodies was observed in all pilomatricomas and in single cases of trichoepithelioma and squamous cell carcinoma showing peculiar signs of matrical differentiation. Moderate increase of beta-catenin nuclear staining was detected in a significant proportion of basal cell carcinomas, Bowen disease, spiroadenomas, and occasionally also in squamous cell carcinomas, but in these neoplasms only a limited fraction of tumor cells accumulated beta-catenin. Molecular analysis revealed that beta-catenin gene mutations are a peculiar feature of skin tumors with matrical differentiation and correlate with a pattern of intense and diffuse beta-catenin nuclear expression. In contrast, adenomatous polyposis coli (APC) and AXIN2 mutations were not involved in skin tumorigenesis. Analysis of Wnt pathway revealed that TCF-1 and MITF-M were selectively induced in the tumor types harboring beta-catenin mutations, indicating that a Wnt/beta-catenin pathway involving TCF-1 and MITF-M is activated in these tumors. Interestingly, high expression levels of TCF-3 were found in basal cell carcinomas and spiroadenomas. TCF-3 is reported to act as a negative modulator of beta-catenin degradation pathway. Thus, the moderate increase of beta-catenin nuclear staining detected in these tumor types might, at least in part, be due to a TCF-3-dependent mechanism. Finally, we found that the presence of beta-catenin mutations significantly correlated with loss of nuclear immunoreactivity for an antibody raised against the N terminus of beta-catenin (alphaABC). Thus, a combined analysis with C terminus-beta-catenin antibodies and alphaABC Ab may represent a powerful investigative approach for the detection of beta-catenin structural alterations.

Chfr inactivation is not associated to chromosomal instability in colon cancers

Numerous observations suggest that chromosome instability is caused by mitotic abnormalities such as errors in the partitioning of chromosomes. Chfr was recently defined as a central component of a new mitotic checkpoint that delays chromosome condensation in response to mitotic stress. Chfr was shown to be frequently inactivated in several human neoplasms, including colon, lung and esophageal cancers. To test whether Chfr inactivation may lead or participate to chromosomal instability (CIN), we analysed the genetic and epigenetic status of the gene in a large panel of primary colon and breast cancers, as well as in colon and breast cancer cell lines displaying either a microsatellite instability or a CIN. Our results confirm that Chfr is frequently inactivated in colon cancers, through a mechanism of hypermethylation of the promoter sequences. In contrast, the loss of Chfr expression appears to be a rare event in breast cancers. Furthermore, our data demonstrate that Chfr inactivation is not associated with CIN in these frequent types of human cancers.

BRAF mutations characterize colon but not gastric cancer with mismatch repair deficiency

Genes from the RAF family are Ras-regulated kinases involved in growth cellular responses. Recently, a V599E hotspot mutation within the BRAF gene was reported in a high percentage of colorectal tumors and significantly associated to defective mismatch repair (MMR). Additionally, BRAF mutations were described only in K-Ras-negative colon carcinomas, suggesting that BRAF/K-Ras activating mutations might be alternative genetic events in colon cancer. We have addressed to what extent the tumorigenic-positive selection exerted by BRAF mutations seen in colorectal MMR-deficient tumors was also involved in the tumorigenesis of gastric cancer. Accordingly, BRAF mutations were detected in 34% (25/74) of colorectal MMR-deficient tumors and in 5% (7/142) of MMR-proficient colorectal cases (P=0.0001). All mutations found in the MSI cases corresponded to the previously reported hotspot V599E. Two D593K and a K600E additional mutations were also detected in three MSS cases. However, only one mutation of BRAF was found within 124 MSS gastric tumors and none in 37 MSI gastric tumors, clearly suggesting that BRAF mutations are not involved in gastric tumorigenesis. Nonetheless, a high incidence of mutations of K-Ras was found within the MSI gastric group of tumors (P=0.0005), suggesting that the activation of K-Ras-dependent pathways contributes to the tumorigenesis of gastric cancers with MMR deficiency. Accordingly, our results show evidences that BRAF mutations characterize colon but not gastric tumors with MMR deficiency and are not involved in the tumorigenesis of gastric cancer of the mutator phenotype pathway.

The CDKN2A database: Integrating allelic variants with evolution, structure, function, and disease association

In this report, we introduce the CDKN2A Database, an online database of germline and somatic variants of the CDKN2A tumor suppressor gene recorded in human disease through the year 2002, annotated with evolutionary, structural, and functional information. The CDKN2A Database improves upon existing resources by: 1) including both somatic mutations and germline variants, thereby adding the perspective of somatic cell carcinogenesis to that of hereditary cancer predisposition; 2) including information that assists with the interpretation of allelic variants, such as other primary data (sequences, structures, alignments, functional measurements, and literature references) and annotations (extensive text, figures, and a tree-based phylogenetic classification); and 3) providing the information in a format that allows a user to either download the database or to easily manipulate it online. We describe the database structure, content, current uses, and potential implications (http://biodesktop.uvm.edu/perl/p16).

Genetic alterations in poorly differentiated endocrine carcinomas of the gastrointestinal tract

BACKGROUND

The molecular pathogenesis of poorly differentiated endocrine carcinomas of the gastrointestinal tract (GI PDECs) remains unclear. It has been suggested that these lesions either originate from multipotent stem cells that also can serve as the origin of nonendocrine adenocarcinomas or arise due to the dedifferentiation of well-differentiated endocrine carcinomas (WDECs).

METHODS

Ten gastric and 9 colorectal PDECs, 9 gastric WDECs, and 12 colorectal carcinomas (CRCs) were analyzed for loss of heterozygosity (LOH) at 11q13 (MEN1), 17p13.1 (p53), 3p14.2 (FHIT), 3p21.3 (RASSF1A), and 18q23 (DCC/DPC4/Smad2), and for immunohistochemical expression of p53, FHIT, Rb, and p16.

RESULTS

PDECs exhibited high fractional allelic loss (FAL; 0.49), with frequent (> 40%) alterations in p53, Rb, MEN1, FHIT, and 18q. No significant differences were found between gastric and colorectal PDECs. Gastric WDECs also exhibited high FAL (0.44), with frequent alterations in Rb and/or p16, MEN1, and 3p21. CRCs exhibited a low level of FAL (0.23), with frequent (> 50%) p16 and p53 alterations. When gastric PDECs and WDECs were compared, substantial similarities were found with respect to FAL (0.42 vs. 0.44) and with respect to individual gene alterations, except in p53, which was consistently altered only in PDECs. CRCs, which were characterized by a lower FAL (0.56 vs. 0.23) and which lacked alterations in both 3p and Rb, were found to be significantly different from colorectal PDECs.

CONCLUSIONS

GI PDECs demonstrated a high level of chromosomal instability; consistent inactivation of both the p53 and p16/Rb pathways; and frequent LOH at 3p (possibly involving FHIT), the MEN1 locus, and 18q. The profile of genetic alterations in PDECs was more consistent with the profile in WDECs than with the profile in CRCs.

Expression of tumor related genes NGX6, NAG-7, BRD7 in gastric and colorectal cancer

AIM: NGX6, NAG-7 and BRD7 genes are tumor related genes, which have been newly cloned by positional candidate cloning strategy. This study was designed to investigate the expression levels of NGX6, NAG-7 and BRD7 genes in human gastric and colorectal cancer tissues, and their corresponding normal tissues, and to investigate whether these genes play a role in the pathogenesis of gastric and colorectal cancers.

METHODS: Reverse transcription-polymerase chain reaction (RT-PCR), dot hybridization and Northern blot analysis were used to compare the expression levels of NGX6, NAG-7 and BRD7 genes in 34 gastric cancer tissues and 34 colorectal cancer tissues with their corresponding normal tissues of the same patients, respectively.

RESULTS: Among the 34 colorectal cancer specimens and the 34 gastric cancer specimens, the expression of NGX6 in 25 colorectal cancer tissues was absent or very weak (73.5%) by RT-PCR analysis. The down-regulation rate of NGX6 in colorectal cancer tissues was significantly higher than that in corresponding normal tissues (26.5%,9/34) (P < 0.005). Moreover, the down-regulation of NGX6 was significantly correlated with lymph node and/or distance metastases. Patients with lymph node and/or distance metastasis had much higher down-regulation rate of NGX6 than patients without metastases (93.8% vs 55.6%, P < 0.05). However no correlation was found between the expression of NGX6 and pathologic type of colorectal cancer in this study, and also the expression of NGX6 did not display any difference between gastric cancer and corresponding normal tissues (58.8% vs 70.6%, P > 0.25). Dot hybridization and Northern blot analysis confirmed the results of RT-PCR. Furthermore, NAG-7 and BRD7 mRNA was not up- or down-regulated in gastric and colorectal cancers compared with their corresponding normal tissues in our study.

CONCLUSION: The down-regulation of NGX6 may be closely associated with tumorigenesis and metastasis of colorectal carcinoma. However, it may not contribute to the development and progression of gastric carcinoma. In addition, the expression levels of NAG-7, and BRD7 did not alter in gastric and colorectal cancers. This seems to suggest that NAG-7 and BRD7 genes may not play a role in gastric and colorectal carcinogenesis.

The Wnt signaling pathway and its role in tumor development

Cancer development depends on the aberrant activation of signal transduction pathways that control cell growth and survival and play important roles in normal embryonic development. This review will focus on one of the most powerful pathways, the canonical Wnt signal transduction cascade, which has been originally described in vertebrate and non-vertebrate embryogenesis and subsequently associated with the development of a multitude of different tumor types, mainly of gastrointestinal origin. In recent years, a variety of novel interacting components and functions have been identified in the Wnt pathway revealing not only the complexity of Wnt signaling but also its potency. Here we will concentrate on the role of the Wnt pathway in cancer development with emphasis placed on the molecular defects known to promote neoplastic transformation in humans and in animal models.

Total loss of MHC class I in colorectal tumors can be explained by two molecular pathways: beta2-microglobulin inactivation in MSI-positive tumors and LMP7/TAP2 downregulation in MSI-negative tumors

The mechanisms that lead to loss of MHC class I expression in different types of tumors are not yet fully known. Accordingly, we studied colorectal carcinomas to elucidate the specific mechanisms of evasion of the T-cell immune response. We selected tumors with total loss of MHC class I expression and studied 124 colorectal carcinomas with immunohistochemical staining and anti-HLA monoclonal antibodies (mAb). Fourteen of 124 (11%) tumors exhibited a phenotype with HLA class I total loss. Microsatellite instability (MSI) analysis was also carried out in the same tumor samples. The expression of beta2-microglobulin (beta2m), HLA-A, B, and C antigens, transporter associated with antigen processing 1 (TAP1), TAP2, low-molecular-weight protein 2 (LMP2), and LMP7 were analyzed using reverse-transcription polymerase chain reaction (RT-PCR) in microdissected tumor samples. Four of 14 microsatellite instability-positive (MSI+) and W6/32 mAb-negative tumors showed biallelic inactivation of beta2m and accumulation of HLA class I heavy chain in the cytoplasm. MSI-negative (MSI-)/W6/32 mAb-negative tumors presented alterations in the expression of components of the antigen processing machinery (APM). Nine of 10 tumor samples showed LMP7 gene downregulation, and four of 10 presented TAP2 dysregulation. This group apparently expressed normal levels of heavy chain and beta2m mRNA. Two major mechanisms in colorectal cancer appear to be responsible for the total loss of MHC surface expression (beta2m mutations and LMP7/TAP2 downregulation) that may contribute to the failure of T lymphocyte recognition during an immune response. The precise identification of the molecular defects that underlie HLA class I abnormalities will have important implications for patients receiving T-cell-based specific immunotherapy.

Analysis of the human APC mutation spectrum in a saccharomyces cerevisiae strain with a mismatch repair defect

Somatic APC mutations in colorectal tumors with an RER phenotype reflect excessive frameshift mutations, especially in simple repetition tracts within the coding sequence. Because this type of mutation is characteristic of cells with a deficient DNA MMR system, the APC mutation signature of RER tumors may be attributable to a defect in the MMR system. However, there is little experimental evidence to prove that the spectrum of mutations and the APC gene distribution are directly influenced by MMR system defects. We therefore examined the mutation spectrum of the MCR of the APC gene after transfection into both MMR-proficient and MMR-deficient yeast strains and compared it with a previously reported human APC mutation database. Small insertions or deletions in mono- or dinucleotide repeats were more common in the MMR-deficient than in the MMR-proficient strain (91.2% vs. 38.1%, Fisher's exact test p < 0.0001). Furthermore, the 2 mutation hot spots, 4385-4394(AG)(5) and 4661-4666(A)(6), found in the yeast system corresponded with those in human tumors. Combining our data with those from human tumors, there appears to be hypermutable mutations in specific simple repetitive sequences within the MCR, which are more prevalent in MMR-deficient cells and RER tumors than in MMR-proficient cells and non-RER tumors. We therefore consider that the differences in the spectra of RER and non-RER tumors are attributable at least in part to the MMR system of the host cells.

CpG island methylation is a common finding in colorectal cancer cell lines

Tumour cell lines are commonly used in colorectal cancer (CRC) research, including studies designed to assess methylation defects. Although many of the known genetic aberrations in CRC cell lines have been comprehensively described, no studies have been performed on their methylation status. In this study, 30 commonly used CRC cell lines as well as seven primary tumours from individuals with hereditary nonpolyposis colorectal cancer (HNPCC) were assessed for methylation at six CpG islands known to be hypermethylated in colorectal cancer: hMLH1, p16, methylated in tumour (MINT-)-1, -2, -12 and -31. The cell lines were also assessed for microsatellite instability (MSI), ploidy status, hMLH1 expression, and mutations in APC and Ki-ras. Methylation was frequently observed at all examined loci in most cell lines, and no differences were observed between germline-derived and sporadic cell lines. Methylation was found at MINT 1 in 63%, MINT 2 in 57%, MINT 12 in 71%, MINT 31 in 53%, p16 in 71%, and hMLH1 in 30% of cell lines. Overall only one cell line, SW1417, did not show methylation at any locus. Methylation was found with equal frequency in MSI and chromosomally unstable lines. MSI was over-represented in the cell lines relative to sporadic CRC, being detected in 47% of cell lines. The rate of codon 13 Ki-ras mutations was also over three times that expected from in vivo studies. We conclude that CpG island hypermethylation, whether acquired in vivo or in culture, is a ubiquitous phenomenon in CRC cell lines. We suggest that CRC cell lines may be only representative of a small subset of real tumours, and this should be taken into account in the use of CRC cell lines for epigenetic studies.

DNA repair defects in colon cancer

Defects in DNA-repair pathways lead to an accumulation of mutations in genomic DNA that result from non-repair or mis-repair of modifications introduced into the DNA by endogenous or exogenous agents or by the malfunction of DNA metabolic pathways. Until recently, only two repair pathways, postreplicative mismatch repair and nucleotide excision repair, have been linked to cancer in mammals, but these have been joined in recent months also by the damage-reversal and base-excision-repair processes, which have been shown to be inactivated, either through mutation or epigenetically, in human cancer.

Apo2L/TRAIL and its death and decoy receptors

Apo2 ligand or tumour necrosis factor-related apoptosis-inducing ligand (Apo2L/TRAIL) is one of the several members of the tumour necrosis factor (TNF) gene superfamily that induce apoptosis through engagement of death receptors (DRs). Apo2L/TRAIL interacts with an unusually complex receptor system of two DRs and three decoys. This protein has garnered intense interest as a potential candidate for cancer therapy because as a trimer it selectively induces apoptosis in many transformed cells but not in normal cells. While much of the early characterisation of Apo2L/TRAIL and its receptors relied on overexpression studies, recent work using untransfected cells has clarified how endogenous proteins transmit apoptotic signals from this ligand. In this review, we focus on the apoptotic signalling pathways stimulated by Apo2L/TRAIL and summarise what is known about its physiological role.

ROCK and Dia have opposing effects on adherens junctions downstream of Rho

Adherens junctions (AJs) are crucial for maintaining the integrity of epithelial tissues and are often disrupted during tumour progression. Rho family proteins have been shown to regulate adherens junctions. We find that activation of the effector kinase ROCK and acto-myosin contraction disrupts AJs downstream of Rho. In contrast, signalling through the Rho effector Dia1 is required to ensure a dynamically stable interface between cells and the maintenance of adherens junction complexes. The ability of Dia1 to regulate the actin network is crucial for the localization of adherens junction components to the cell periphery.

Translational oncogenomics: toward rational therapeutic decision-making

The wealth of data in basic science and translational research may often represent a conundrum for clinical oncologists, who need to selectively consider this abundant information and translate it into therapeutic decisions. For the sake of simplicity, we have classified the multiplicity of genetic abnormalities in five repertoires that are rapidly assessable and useful for stratification in clinical trials: allelic imbalance, aberrant promoter methylation, gene mRNA overexpression, microtubule alterations, and polymorphisms. Allelic imbalance refers to chromosomal instability, which is a major feature of cancer, and innovative techniques used in colorectal cancer should also be implemented in lung cancer. Epigenetic changes (variations in transcription levels) have been extensively studied in non-small cell lung cancer. Methylation techniques have shown that these epigenetic changes commonly occur at the same frequency in numerous genes, both well-known ( FHIT, APC, p16 ) and recently discovered ( TMS1, RASSF1 ) in non-small cell lung cancer and in breast cancer. Innovative techniques like quantitative polymerase chain reaction can determine gene expression profiles, mainly overexpression of mRNAs, which may be related to resistance to specific cytotoxic drugs. In the near future, we hope these profiles can be used to individualize chemotherapy. Multiple microtubule alterations related to overexpression of different genes can also be used to predict response to taxanes and Vinca alkaloids. Finally, the assessment of polymorphisms could enable us to understand their functional consequences in chemotherapy response.

DNA mismatch repair defects: role in colorectal carcinogenesis

The inactivation of the DNA mismah repair (MMR) system, which is associated with the predisposition to the hereditary non-polyposis colorectal cancer (HNPCC), has also been documented in nearly 20% of the sporadic colorectal cancers. These tumors are characterized by a high frequency of microsatellite instability (MSI(+) phenotype), resulting from the accumulation of small insertions or deletions that frequently arise during replication of these short repeated sequences. A germline mutation of one of the two major MMR genes (hMSH2 or hMLH1) is found in half to two-thirds of the patients with HNPCC, whereas in sporadic cases hypermethylation of the hMLH1 promoter is the major cause of the MMR defect. Germline mutations in hMSH6 are rare and rather confer predisposition to late-onset familial colorectal cancer, and frequent extracolonic tumors. Yet, the genetic background of a number of HNPCC patients remains unexplained, indicating that other genes participate in MMR and play important roles in cancer susceptibility. The tumor-suppressor genes that are potential targets for the MSI-driven mutations because they contain hypermutable repeated sequences are likely to contribute to the etiology and tissue specificity of the MSI-associated carcinogenesis. Because the prognosis and the chemosensitivity of the MSI(+) colorectal tumors differ from those without instability, the determination of the MSI phenotype is expected to improve the clinical management of patients. This review gives an overview of various aspects of the biochemistry and genetics of the DNA mismah repair system, with particular emphasis in its role in colorectal carcinogenesis.